A variety of conventional secondary alkaline cells mainly composed of nickel-cadmium closed type cells are known. Nevertheless, these conventional secondary alkaline cells are not sufficient in the flat discharge characteristic or cannot stably discharge cell voltage under severe environmental conditions in encountered extremely cold and hot regions, or cannot fully sustain sufficient rounds of repeated charge and discharge, and as a result, these conventional secondary alkaline cells cannot fully satisfy the desired requirements of such cells.
To improve the overall performance characteristics of the secondary alkaline cells, the cell manufacturers have attempted to improve the separator by diminishing the electrical resistance of the separator, improving by the resistivity against alkali and oxidation, by minimizing the residual gas (O.sub.2), by improving the retentivity of the electrolytic solution, and by preventing the electrode-plate material from generating dendrite, as means for achieving the aforesaid object.
For example, regarding the nickel-zinc closed type secondary alkaline cell, Japanese Patent Kokai Koho No. 165572/1980 proposes a zinc-alkaline secondary cell featuring the an improved cycle-life characteristic by introducing a composite separator composed of a fluorinated resin containing finely pulverized oxide like TiO.sub.2, ZrO.sub.2, or HfO.sub.2 through 200 mesh filter and a layer of non-woven fabric.
Although application of these means certainly improves the performance characteristics of the secondary alkaline cells to some extent, further improvement is still demanded.
In order to further improve the cell performance characteristics, the present inventors conducted extensive studies and trials. As a result, the present inventors have successfully improved the characteristics of the separator used for preparing a second alkaline cell having improved retentivity of electrolytic solution, improved resistivity against alkali, decreased residual gas (O.sub.2) generated during charge, reduced electrical resistance of the separator, improved resistivity against oxidation, and preventing dendrite from occurring, by effectively applying titanium dioxide having a specific primary particle diameter, a specific crystal form, and a specific surface area, from such. The titanium dioxide constitutionally having a hydrophillic property and a distinctly outstanding resistivity against alkali, oxidation, and chemicals, and the least electrical resistance. As a result of successful trials, the present inventors confirmed that the performance characteristics of the secondary alkaline cell could significantly be improved, and based on their acquired knowledge, the present inventors have eventually achieved the invention.
On the other hand, alkaline-manganese primary cells are commercially available as primary alkaline cells. However, these conventional alkaline-manganese primary cells are still defective in that the total discharge duration is very short, and yet, they cannot stably discharge current in severe environmental conditions such as in very cold and very hot regions, and these conventional alkaline-manganese primary cells cause the alkaline solution (functioning as an electrolytic solution) to externally leak, and thus, these cells do not always have satisfactory performance characteristics.
There is a positive demand for improving the discharge characteristic by expanding the cell capacity, diminishing the internal resistance, improving the discharge temperature characteristic, and for improving the retentivity of electrolytic solution by improving the prevention of the leakage of electrolytic solution altogether.
As an effective measure to improve the discharge characteristic and the retentivity of electrolytic solution, trials were undertaken to improve the overall characteristics of the separator, which is disposed between anode and the cathode, so that the requirements including the expansion of the cell capacity, reduction of the internal resistance, improvement of the discharge temperature characteristic and prevention of the leakage of electrolytic solution can be satisfied.
For example, Japanese Patent Kokoku Koho No. 4048/1979 proposes a method of improving the electrical characteristic of a primary alkaline cell by applying a separator composed of a composite layer made of polypropylene/titanium dioxide/polystylene. This composite layer is prepared by initially coating titanium dioxide over a thin film of polypropylene, followed by a drying process, applying a coating of polystylene solution over the dried layer, and conducting a final drying. Furthermore, Japanese Patent Kokoku Koho No. 30939/1981 proposes the provision of a separator which is chemically stable to the electrolytic solution and to the anode by producing the cell separator by filling a polymer of ethylene and acrylic acid into fine holes of the porous film mainly consisting of fine particles made from an inorganic compound which is insoluble in electrolytic solution.
According to the aforementioned prior art, the characteristics of the separator can be improved, and based on this, the discharge characteristic and the retentivity of electrolytic solution can also be improved. Nevertheless, further improvement of these requirements is still demanded.
In order to further improve the discharge characteristic and the retentivity of an electrolytic solution, the inventors continuously conducted extensive studies and trials. As a result, the inventors discovered that the characteristics of the separator usable for preparing a primary alkaline cell could significantly be improved by effectively applying titanium dioxide having a predetermined primary particle diameter, a specific crystal form, and a predetermined specific surface area by selecting it from such titanium dioxide inherently having a hydrophillic property and having outstanding resistivity against alkali, oxidation, and chemicals, and having the least electrical resistance. Based on the knowledge thus acquired, the present inventors have successfully achieved the invention.